Repairing electrolytic cells

ABSTRACT

Holes which develop in the base of an electrolysis chamber in an electrolytic cell and expose a metal member in the said base to contact by liquid material in the chamber are repaired during operation of the cell by feeding into the liquid a granular material which settles through the liquid and plugs the hole.

I llmted States Patent 1191 1111 3,766,025 Wir 14 1 Oct. 16, 1973 [54]REPAIRING ELECTROLYTIC CELLS 3,428,545 2/l969 Johnson 204/243 R X3,457,149 7/1969 Johnson 204/243 R X [75] Invent Gasm Albermarle,3,514,520 5/1970 Bacchiega et a] 204/243 R x [73] Assignee: AluminumCompany of America,

Pittsburgh, Pa. Primary Examiner-John l-l. Mack Assistant Examiner-D. R.Valentine [22] June 1972 Attorney-Edward B. Foote 21 Appl. No.: 268,180

[57] ABSTRACT [52] US. Cl. 204/67 H le whi h velop in he base of anelectrolysis [51] Int. Cl C22d 3/12 c m r in an le rolytic ll and expo ea metal [58] Field of Search 204/67 m m er in h i ba to contac y liquidm erial in the chamber are repaired during operation of the [56]References Cited cell by feeding into the liquid a granular materialUNITED STATES PATENTS which settles through the liquid and plugs thehole.

2,971,899 2/1961 l-lannink et a1. 204/243 R X 4 Claims, N0 DrawingsREPAIRING ELECTROLYTIC CELLS This invention relates to repairing theelectrolysis chamber of an electrolytic cell, and relates particularlyto procedures for effecting such repair during operation of the cell.

BACKGROUND OF THE INVENTION Aluminum is conventionally producedcommercially by the well-known Hall-Heroult process, in whichelectrolysis of alumina dissolved in molten fluorides principallycryolite is carried out in electrolysis chambers having an electricallyconductive lining surrounded by insulation and a metal sheath, therebeing one or more anodes disposed above the cell and extending into thealumina-fluoride electrolytic bath. A layer of molten aluminum in thecell rests on the lower portion of the electrolysis chamber and acts asthe cell cathode. The above-mentioned lining ordinarily consists ofbaked carbon, either in monolithic form or in blocks bonded togetherwith a carbonaceous binder. Metal (e.g., iron, steel or copper)collector bars embedded in the carbon lining and connected externally tothe cathode bus conduct electric current from the cell. U. S. Pat. Nos.2,528,905, 2,593,751, 2,874,103, 2,874,110, 3,236,753, 3,494,851 and3,514,520 disclose cells of the type referred to above.

The inventionwill be described below in connection with electrolyticcells for use in the Hall-Heroult process for the production ofaluminum, although it is also applicable to cells for other electrolyticprocesses, for example, the well-known three-layer process forelectrolytic purification of aluminum. U. S. Pat. Nos. 1,534,316,1,534,317 and 2,582,661 disclose the threelayer process and cells of thetype used in operation of it.

As is pointed out in Formation of Potholes in Bottom Linings of HallCells, by Dell, Peterson and rumble, Journal of Metals, September 1968,pages 55-58, a common problem encountered in the operation ofelectrolytic cells for the production of aluminum is the gradualformation of portholes" in the lower portion of the electrolysischamber. Such holes are eroded areas, shaped roughly like invertedcones, which develop downward from the molten pool of aluminum restingon the electrolysis chamber base, and gradually become deep enough topenetrate to a metal collector bar embedded in that base, permittingmolten aluminum to contact the bar and dissolve metal therefrom, withresultant contamination of the aluminum, as well as impairment of thecurrent efficiency of the cell. The collector bar area thus contacted bythe molten aluminum is referred to herein as the exposed area of thebar.

Penetration of a pothole to a metal collector bar embedded in the baseof the electrolysis chamber can be detected by periodic analysis of themolten aluminum in the cell to determine whether it has beencontaminated by collector bar metal. When such contamination isobserved, it has been the practice to discontinue operation of the cell,drain the electrolytic bath and molten aluminum from it, and replace orrepair the electrolysis chamber, all at substantial expense andinterference with production of metal in the cell.

The Invention It is the object of this invention to provide a convenientand economical method of repairing holes in the base of an electrolysischamber of an electrolytic cell during continued operation of the cell.

In accordance with the invention, upon finding tha a hole in the base ofthe electrolysis chamber of an electrolytic cell has penetrated to ametal bar embedded in that base, the location of the hole isascertained. This may be done by probing the bottom of the electrolysischamber with a metal rod inserted downward through the electrolytic bathlayer and the molten aluminum layer beneath it. Locating the hole inthat manner can be facilitated by first identifying the particular baraffected, which can be accomplished by measuring the current loading ofthe respective collector bars in the lining with a suitable ammeter. Acurrent loading outside normal range on a particular collector bar is anindication that it is the bar with which molten aluminum in the chamberis in contact. The location of the hole can then be determined byprobing the bottom of the electrolysis chamber in the area adjacent theaffected collector bar.

When the hole in the lining has been thus located, granular materialhaving sufficient density for it to settle through the electrolytic bathand the molten aluminum layer beneath it is fed into the bath in thearea over the hole, whereupon the granular material settles into thehole, Sufficient granular material is fed into the hole in this mannerto cover the exposed area of the collector bar thoroughly and preventfurther access of molten aluminum thereto, thereby forestalling furtherattack on the metal bar by the aluminum. Following introduction of thedesired amount of granular material into the hole, it is desirable topack it securely in place by tamping it with a metal rod. Shielding ofthe granular material by the liquid metal layer prevents or minimizesits dissolution in the molten electrolytic bath.

The granular material employed may be in the form of balls, chips,pellets, or the like of a material which is not readily soluble in, ormelted by, the electrolytic bath or the molten aluminum. Pellets ofalumina are particularly suitable for the purpose, since they do notintroduce undesirable impurities into the bath or the aluminum, and arecapable of withstanding prolonged contact with the molten aluminum.Pellets of alpha alumina are preferred, since alpha alumina is lessreadily soluble in the electrolytic bath than other crystalline oramorphous forms of alumina. However, granules of such other refractorymaterials such as silicon carbide and boron nitride, can also be used incarrying out the invention.

The above-mentioned steps of locating the hole in the base of theelectrolysis chamber, feeding granular As an example of the invention,in producing aluminum in an electrolytic cell with an electrolysischamber having a carbon bottom and side walls, horizontally disposediron collector bars embedded in the chamber bottom, and a number ofpre-baked carbon electrodes suspended in a bath of molten fluorides anddissolved alumina, upon detecting an increase in the iron content ofaluminum produced in the cell to about 0.8 percent by weight from anormal level of about 0.2 percent, the current loading of the variousiron collector bars was checked with an ammeter and it was found thatthe loading of one of the bars was unusually high. Cell anodes over thatcollector bar were removed temporarily from the electrolytic bath,without discontinuing feeding electric current to the cell throughremaining anodes of the cell, and upon probing the electrolysis chamberfloor in the area adjacent that bar with a steel rod inserted throughthe molten layers of electrolytic bath and aluminum, a pothole extendingto the collector bar was located in the chamber floor.

Small balls (approximately three-fourths inch in diameter) of alphaalumina, with a specific gravity of about 4, and mixed with fineparticles of such alpha alumina, were poured into the electrolytic bathin the area above the pothole. The mixture settled into the pothole,displacing the molten aluminum therein and covering the exposed area ofthe collector bar. The mixture in the hole was next tamped into placewith a steel rod, after which the above-mentioned removed anodes wereput back into place and were again used to supply current to the cell.After two weeks of continued operation of the cell in a normal manner,the molten aluminum in the cell was analyzed to determine its ironcontent, which content was found to be about 0.1 percent. The currentloading of the above-mentioned collector bar at that time was at normalrange.

I claim:

1. In the operation of an electrolytic cell having an electrolysischamber with a bottom on which liquid is supported, the said bottomhaving current-conducting metal members embedded therein beneath thesaid liquid, the method of repairing a hole in the said chamber bottomwhich exposes an area of one of said members to contact by said liquid,which comprises the steps of determining the location of the said hole,and feeding into the liquid in the area over the hole sufficientgranular material which settles into the said hole to cover the saidexposed area and prevent access thereto by said liquid.

2. The method of claim 1 in which the location of the said hole isdetermined by probing the said bottom in the area adjacent acurrent-conducting member exhibiting a higher current loading thannormal therefor.

3. The method in accordance with claim 1, in which the cell is one forthe electrolytic reduction of alumina to aluminum in a fused fluoridebath, and the said liquid consists of molten aluminum.

4. The method in accordance with claim 1, in which the said granularmaterial consists of alumina.

2. The method of claim 1 in which the location of the said hole isdetermined by probing the said bottom in the area adjacent acurrent-conducting member exhibiting a higher current loading thannormal therefor.
 3. The method in accordance with claim 1, in which thecell is one for the electrolytic reduction of alumina to aluminum in afused fluoride bath, and the said liquid consists of molten aluminum. 4.The method in accordance with claim 1, in which the said granularmaterial consists of alumina.